This invention relates to methods and systems for co-molding thermoplastic materials such as thermoplastic composite materials with thermoplastic sandwich materials and articles produced thereby.
Manufacturing Thermoplastic Sandwich Materials (i.e. TSM)
European patent EP 0 649 736 B1 explains the principle of molding substantially flat parts out of thermoplastic sandwich material (TSM). The part is made in a single stage by pressing in a cold mold, at a pressure in the range of 10 bars to 30 bars, a stack consisting of at least a first top skin layer of stampable reinforced thermoplastics material, a cellular or honeycomb core of thermoplastics material and a second bottom skin layer of stampable reinforced thermoplastics material. The axes of the cells of the cellular core are generally oriented perpendicular to the skin layers. The skin layers and core are previously heated outside the mold to a softening temperature.
As described in French patent application 97 06666, it is possible to make a fixation or fixing with a hole by piercing the thermoplastic sandwich material in the mold with a sharp tool.
As described in French Patent No. 2 751 914, it is possible to reinforce the radial resistance of a fixation in such a TSM part by completely crushing the central honeycomb.
European patent application EP 0 903 216 A2 discloses over-molding a steel insert with claws that are anchored in the thermoplastic sandwich material to distribute the load on the insert.
Drawbacks Of The Steel Insert With Claws
The results are very good for shear resistance of the fixation or insert if the skins use woven fibers, but the results are poor if the skins use non-woven fibers;
It is necessary to invest in an additional tool to stamp the steel insert;
It is necessary to treat the steel insert to avoid corrosion; and
The part is not tight anymore because of the through hole and because of the claws that create holes in the skins.
In the case of fixations or inserts that do not include through holes, it is possible to add such inserts in secondary operations:
Vibration welding, heat welding, and ultrasonic welding can be used to weld inserts made of the same thermoplastic as the skins;
Clips can be used in a shape made in the sandwich part;
Semi-structural gluing with low mechanical resistance if the surface is not prepared for good bonding;
Structural adhesion to assemble the sandwich part with another part; in that case, an outer surface of the sandwich part as well as the other part surface must be treated with an adhesion promoter to improve the gluing between the two parts; and
Mechanical fastening with inserts such as rivets.
These operations represent an additional cost due to additional workforce and machines.
Also, the recyclability of the part is not as good if other materials such as inserts are used.
In conclusion and before the present invention, there was no solution to integrate a TSM part with another part directly during the molding process and without drilling a hole as a preliminary step, if high loads are to be applied to the resulting part.
Solutions Developed For Sandwich Panels Used In Aeronautics, Trains, and Other Industries
The Hexcel Company has developed a variety of ways to mechanically fasten fixings to their composite panels. However, each of the methods are time-consuming and costly.
Molding of a Thermoplastic Composite Material Such As GMT (Flowable PP-Glass Mat) Material and Thermoplastic Sandwich Material
The technique of molding GMT (Glass Mat Thermoplastics) materials uses a thermoplastic (polypropylene is preferred) and glass mats just as sandwich materials but the molding process of GMT is at a relatively high pressure (i.e. pressures of 100xe2x88x92200 bars (107Nxe2x88x922xc3x97107N)) whereas thermoplastic sandwich materials uses a pressure 10 to 20 times lower (i.e. 10 to 20 bars (106Nxe2x88x922xc3x97106N)).
The GMT process is a flow pressure process wherein the GMT material flows under pressure in a mold and allows the production of parts as thin as 2 mm and with the possible integration of the following features:
Ribs can be molded to improve stiffness; and
Inserts can be over-molded.
The technique of molding thermoplastic sandwich material can be used to mold with a low pressure of 10 to 20 bars semi-structural or structural parts, possibly with a 3D deep-draw, while maintaining a constant thickness as described in the above-noted copending U.S. application. If the constant thickness is maintained, the stiffness/weight ratio of the material is very high as compared to GMT.
An object of the present invention is to provide a method and system for co-molding a thermoplastic material with a thermoplastic sandwich material and article produced thereby wherein the thermoplastic material portion of the article allows the article to be easily attached to another article such as a car body.
Another object of the present invention is to provide a method and system for co-molding a thermoplastic material with a thermoplastic sandwich material and article produced thereby wherein the thermoplastic sandwich material is integrated or fused with the thermoplastic material during the molding process so that the article can withstand high loads.
Yet another object of the present invention is to provide a method and system for co-molding a thermoplastic material with a thermoplastic sandwich material and article produced thereby wherein the article takes full advantage of the properties of the two materials.
In carrying out the above objects and other objects of the present invention, a method for co-molding a thermoplastic material with a thermoplastic sandwich material to form a thermoplastic sandwich article having a thermoplastic inner portion is provided. The method includes positioning a blank of thermoplastic sandwich material having a cellular core over a female die having an article-defining cavity defined by inner surfaces of the female die. The method also includes placing a predetermined amount of thermoplastic material on the blank of thermoplastic sandwich material. An inner portion of the blank is forced into the female die along a substantially vertical axis and against the inner surfaces of the female die. A peripheral edge portion of the sandwich material is crushed about the thermoplastic material. The thermoplastic material and the sandwich material are forced within the peripheral edge portion together at a temperature and pressure sufficient to fuse the materials together to form a thermoplastic inner portion of the sandwich material. The method further includes removing the sandwich material with the thermoplastic inner portion from the female die, and removing any excess material from the periphery of the sandwich material to form the article.
In one embodiment, the step of forcing obtains a deep-drawn sandwich material. In this embodiment, at least one outer portion of the blank is clamped immediately adjacent the female die to guide the at least one outer portion of the blank to travel into the article-defining cavity at an acute angle with respect to the vertical axis during the step of forcing so that thickness of at least one side wall of the deep-drawn sandwich material is substantially the same as thickness of the blank of thermoplastic sandwich material and so that the deep-drawn sandwich material does not significantly stretch or tear during the step of forcing. The resulting article is a deep-drawn article.
The predetermined amount of thermoplastic material may be a blank of the thermoplastic material, an extrusion of the thermoplastic material, or an injection of the thermoplastic material.
The step of forcing the inner portion may include the step of stamping.
The step of clamping may be performed with a clamping force which increases during the step of forcing the inner portion.
The step of clamping may be performed at a plurality of spaced outer portions of the blank immediately adjacent the female die.
The step of forcing the inner portion preferably is performed in a single stamping stage.
The method further comprises the step of placing a nut or stud within the article-defining cavity so that the thermoplastic material over-molds the nut or stud.
Further in carrying out the above objects and other objects of the present invention, a system for co-molding a thermoplastic material with a thermoplastic sandwich material to form a thermoplastic sandwich article having a thermoplastic inner portion is provided. The system includes a female die having an article-defining cavity defined by inner surfaces of the female die. A crushing device is provided for crushing a peripheral edge portion of a blank of sandwich material having a cellular core about a predetermined amount of thermoplastic material placed on the blank. A male die is provided for forcing the thermoplastic material and the sandwich material within the peripheral edge portion together at a temperature and pressure sufficient to fuse the materials together to form a thermoplastic inner portion of the sandwich material.
The system may further include an outer male die for forcing an inner portion of the blank of thermoplastic sandwich material and the predetermined amount of thermoplastic material placed on the blank into the female die along a substantially vertical axis and against the inner surfaces of the female die to obtain deep-drawn sandwich material.
The system may also further include a clamping mechanism for clamping at least one outer portion of the blank immediately adjacent the female die to guide the at least one outer portion of the blank to travel into the article-defining cavity at an acute angle with respect to the vertical axis during forcing of the inner portion of the blank into the female die so that thickness of at least one side wall of the deep-drawn sandwich material is substantially the same as thickness of the blank of thermoplastic sandwich material and so that the deep-drawn sandwich material does not significantly stretch or tear as the inner portion of the blank is forced into the female die.
A stamping press may be provided for forcing the outer male die into the female die.
A hydraulic jack may be provided for forcing the male die into the female die.
The thermoplastic material may be a thermoplastic composite material.
The predetermined amount of thermoplastic material may be a blank of thermoplastic composite material.
An extruder may be provided for extruding the predetermined amount of thermoplastic material.
An injector may be provided for injecting the predetermined amount of thermoplastic material.
The clamping mechanism may be spring-loaded so that the clamping mechanism exerts a clamping force at the at least one outer portion of the blank, the clamping force increasing as the inner portion of the blank is forced into the female die.
The clamping mechanism preferably includes a clamping assembly mounted to move with the male die and having at least one spring which compresses as the inner portion of the blank is forced into the female die.
The clamping mechanism preferably includes a clamping assembly and a counter clamping assembly for clamping a plurality of spaced outer portions of the blank immediately adjacent the female die so that a substantially constant distance is maintained between the clamping assembly and the counter clamping assembly as the inner portion of the blank is forced into the female die. Each of the assemblies typically includes a plurality of elongated clamping surfaces for clamping the plurality of spaced outer portions of the blank. Each elongated clamping surface is inclined at the acute angle with respect to the vertical axis.
The inner surfaces of the female die may define a plurality of corners which correspond to spaces between the plurality of spaced outer portions. The spaces are sized to permit thermoplastic sandwich material of the blank to move therein as the inner portion of the blank is forced into the female die.
Preferably, in both the method and system the peripheral edge portion of the sandwich material completely encloses the thermoplastic material.
Co-molding a thermoplastic composite material such as GMT and a thermoplastic sandwich material combines the advantages of the sandwich material (stiffness/weight ratio, low pressure process) and GMT (flow process to mold complex shapes such as ribs but also the possibility to mold a part without excess material where the edge of the part is molded by flow).
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.